As shown in FIG. 8, a tool control system used in a hydraulic circuit of a work machine employs solenoid-operated variable pressure relief valves 5 that are provided on external output lines 4 through which hydraulic oil discharged from variable delivery pumps 1 is controlled by a control valve 2 and fed to an attachment tool 3. Thus provided, the solenoid-operated variable pressure relief valves 5 serve as external relief valves. As shown in FIG. 9, each solenoid-operated variable pressure relief valve 5 is designed to set relief pressure in response to a command electric current value A0-A7. Therefore, as shown in FIG. 8, the tool control system is designed so that a machine controller 7 outputs to each solenoid-operated variable pressure relief valve 5 an electric current value selected from among the command electric current values A0-A7 based on a relief pressure that has been set by means of a monitor 6 installed in the cab of a construction machine or the like. Thus, the set relief pressure for each solenoid-operated variable pressure relief valve 5 can be changed easily without the necessity of manually turning a screw that is attached to the relief valve.
With the conventional tool control system, however, once a relief pressure is set based on a given electric current value, the relief valve is controlled by the constant current that corresponds to the set relief pressure. Therefore, as shown in FIG. 9, when the flow rate of the hydraulic oil passing through the relief valve increases, there arise pressure override characteristics, in other words an increase in pressure caused by valve resistance, resulting in a difference between the set relief pressure and the actual pressure.
On the other hand, examples of hydraulic control of such apparatuses as a hydraulic press include a pressure control shown in FIG. 10, wherein a control device 9 compensates for pressure override characteristics of a solenoid-operated variable pressure relief valve 10 based on a commanded pump flow rate represented by a signal that transmits a command to control the flow rate of hydraulic oil discharged from a variable delivery pump 8 (e. g. see Japanese Laid-open Patent Publication No. 5-146900 (“JP '900”) (p 2, and FIG. 1)).
The technology for pressure override compensation described in JP '900 is for compensating for pressure override characteristics of a solenoid-operated variable pressure relief valve based on a commanded pump flow rate. Therefore, when employed in a machine that is provided with a plurality of hydraulic actuators, the technology is incapable of compensating for pressure override of relief pressure for a specific hydraulic actuator with high accuracy.
In order to solve the above problem, an object of the invention is to improve accuracy of relief pressure with respect to a set relief pressure of a solenoid-operated variable pressure relief valve that is provided for controlling pressure of a specific hydraulic actuator.